Traditionally, deliquescent compounds, usually salts, are used to reduce relative humidity in a closed environment. It is well known that different compounds have varying affinity for moisture. For example, each deliquescent compound has a characteristic capacity for moisture adsorption and a characteristic equilibrium relative humidity (ERH) when hydrated.
Desiccants can be considered humidity controllers in that they have been used to completely (or almost completely) remove all water vapor from the air from a closed system. An effective desiccant in sufficient quantity will adsorb water vapor from the air in a package, lowering the equilibrium relative humidity (ERH) to the point where condensation will no longer occur, or to a point where the threshold ERH within a sealed package or system is never exceeded under the conditions to which the package or system will be exposed. A larger quantity of an effective desiccant will reduce water vapor in a closed system well below the dew point to where the relative humidity of the system matches the ERH of the desiccant at its current degree of hydration.
However, there are applications (food, pharmaceutical, analytical, medical diagnostics, to name a few) where a desiccant is not the best alternative. In such cases, ERH of the product must be maintained at some specific intermediate level, rather than near zero. A common and economical method of controlling humidity in moist environments is the use of humectant compounds. Inorganic salts are the most effective and are most often used, although many deliquescent or hydratable compounds can be used in an appropriate system. These compounds have an affinity for water that regulates the water vapor pressure in the atmosphere within a closed vessel or chamber. In essence, such compounds will adsorb moisture until they go completely into solution. As this occurs, a mixture of solid salt and salt solution will coexist. This solution will be saturated and will have an ERH characteristic of the particular salt or compound used.
The specific humectant in such cases is chosen based on the desired equilibrium relative humidity (ERH). The salt may be single in nature, such as lithium chloride. A mixture of two salts may also be used. As an example, a solution of potassium carbonate has a relative humidity of about 43%. Therefore, a solution of potassium carbonate with excess undissolved crystals of potassium carbonate will maintain a constant relative humidity of approximately 43%. If the relative humidity begins to rise above 43%, the salt solution would pick up moisture from the environment thus lowering the relative humidity closer to 43%. Conversely, if the relative humidity begins to fall below 43%, the solution would release moisture until the surrounding environment reaches approximately 43%. The ERH values for different saturated aqueous salt solutions can vary from 11% to 98%.